U.S. patent application number 13/478199 was filed with the patent office on 2012-11-29 for optical information processing device and tracking control method thereof.
This patent application is currently assigned to Hitachi Media Electronics Co., Ltd.. Invention is credited to Kunikazu OHNISHI.
Application Number | 20120300602 13/478199 |
Document ID | / |
Family ID | 47199423 |
Filed Date | 2012-11-29 |
United States Patent
Application |
20120300602 |
Kind Code |
A1 |
OHNISHI; Kunikazu |
November 29, 2012 |
OPTICAL INFORMATION PROCESSING DEVICE AND TRACKING CONTROL METHOD
THEREOF
Abstract
An optical information device used with a grooveless multilayer
disc including multiple recording layers used to record and
reproduce information signals and a guide layer dedicated to detect
tracking error signals (TES) can always stably detect the TESs when
the distance between the recording layer and guide layer varies due
to selection of a target recording layer. For example, a plurality
of light spots for detecting the TESs are formed by a holographic
grating on the guide layer, but are defocused with respect to each
other. The TESs are detected individually from the respective light
spots. The TESs are subjected to an addition operation to be a
signal for tracking control, thereby extraordinary increasing the
defocus dynamic range of the TESs.
Inventors: |
OHNISHI; Kunikazu;
(Yokosuka, JP) |
Assignee: |
Hitachi Media Electronics Co.,
Ltd.
|
Family ID: |
47199423 |
Appl. No.: |
13/478199 |
Filed: |
May 23, 2012 |
Current U.S.
Class: |
369/44.26 ;
G9B/7.062 |
Current CPC
Class: |
G11B 7/2405 20130101;
G11B 7/0903 20130101; G11B 7/0906 20130101; G11B 2007/0013
20130101; G11B 7/0938 20130101 |
Class at
Publication: |
369/44.26 ;
G9B/7.062 |
International
Class: |
G11B 7/09 20060101
G11B007/09 |
Foreign Application Data
Date |
Code |
Application Number |
May 26, 2011 |
JP |
2011-117765 |
Claims
1. An optical information processing device compatible with an
optical disc, as a recording medium, including at least three
recording layers and a guide layer with a predetermined guide
groove or pit arrays formed thereon, the optical information
processing device comprising: a first light source generating a
first light beam used to reproduce an information signal recorded
on the recording layers or to record an information signal on the
recording layers; a second light source generating a second light
beam used to detect the guide groove or pit arrays provided on the
guide layer; an optical element irradiated with the second light
beam and splitting the second light beam into at least two light
beams to form their light converging spots at positions a
predetermined distance away from each other in an optical axis
direction; an objective lens irradiated with the first light beam
and the light beams split from the second light beam by the optical
element, converging the first light beam on any one of the
plurality of recording layers, and applying the light beams split
from the second light beam by the optical element on the guide
layer; a photodetector separately detecting a reflected light beam
of the first light beam, the reflected light beam coming from the
recording layer, and at least two reflected light beams of the
light beams split from the second light beam by the optical
element, the reflected light beams coming from the guide layer; a
signal reproducing circuit performing reproduction process of the
information signal recorded on the recording layer based on a
detection signal detected by the photodetector, the detection
signal being detected from the reflected light beam of the first
light beam from the recording layer; and a tracking error signal
generating circuit controlling the tracking position of the
objective lens relative to the recording layer based on detection
signals detected by the photodetector, the detection signals being
detected from the at least two reflected light beams of the second
light beam from the guide layer.
2. The optical information processing device according to claim 1,
wherein the tracking error signal generating circuit is adapted to
control the tracking position of the objective lens relative to the
recording layer by adding a plurality of detection signals obtained
from light beams detected by the photodetector, the light beams
being obtained by splitting the second light beam by the optical
element.
3. The optical information processing device according to claim 1,
wherein the optical element is a holographic grating with a grating
pattern of unequally-spaced curved grooves.
4. The optical information processing device according to claim 3,
wherein the light beams split by the optical element are 0 order
light (transmitted light), +1 order diffracted light and -1 order
diffracted light of the second light beam.
5. A tracking control method used in an optical information
processing device compatible with an optical disc, as a recording
medium, including at least three recording layers and a guide layer
with a predetermined guide groove or pit arrays formed thereon, the
optical information processing device performing tracking control
for the recording layers based on light reflected from the optical
disc, the method comprising: controlling tracking positions
relative to the recording layer based on light beams reflected from
the guide layer, the reflected light beams being derived from a
light beam that is generated by a light source, split into at least
two and applied to the guide layer.
Description
INCORPORATION BY REFERENCE
[0001] This application relates to and claims priority from
Japanese Patent Application No 2011-117765 filed on May 26, 2011,
the entire disclosure of which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] (1) Field of the Invention
[0003] The present invention relates to optical information devices
that optically record information signals on optical information
recording media (hereinafter, referred to as optical discs) or
reproduce information signals recorded on the optical discs, and
more particularly, to an optical information processing device
suitable for recording or reproduction of information signals on a
multilayer optical disc with a plurality of recording layers
stacked each other, and a tracking control method adopted by the
optical information processing device.
[0004] (2) Description of the Related Art
[0005] Currently commercially available optical discs include, for
example, DVDs with a storage capacity of 4.7 GB (Giga Byte) on one
layer of one side and Blu-ray Discs having a larger storage
capacity.
[0006] Recently proposed multilayer optical discs have, in addition
to one or two information signal recording layers (hereinafter, the
information signal recording layer is simply referred to as
"recording layer", for clarity), a stack of three or more recording
layers to increase the storage capacity. Such multilayer optical
discs are rapidly proceeding toward standardization and practical
utilization.
[0007] In order to achieve a large capacity multilayer optical
disc, attention is being given to a so-called "grooveless
multilayer disc" including a disc layer used exclusively for
tracking control (hereinafter, the disc layer used exclusively for
tracking control is simply referred to as "guide layer") in
addition to the recording layers. The guide layer is used for
detecting a tracking error signal that controls a light spot that
is used for recording/reproduction and converged on a recording
layer. Only the guide layer has a continuous guide groove formed
thereon to detect the tracking error signal.
[0008] By the way, an optical pickup suitable for recording and
reproduction of such a grooveless multilayer disc is disclosed, for
example, in Japanese Patent Application Laid-Open No. 2003-067939
(PTL 1). The pickup in the disclosure adopts a tracking control
method in which light beams are converged to form independent light
spots on the recording layer and the guide layer, and tracking
error signals are detected from the light spots converged on the
guide layer (hereinafter, the light spots are referred to as "light
converging spots G" for clarity) to perform tracking control of the
light converging spots G based on the tracking error signals, while
performing tracking control of a signal light spot converged on the
recording layer (hereinafter, the light spot is referred to as
"light converging spot R" for clarity) so as to follow the light
converging spots G.
SUMMARY OF THE INVENTION
[0009] The optical pickup compatible with the grooveless multilayer
disc of course needs to form the light converging spot G of
diffraction limited size on the guide layer and the light
converging spot R of diffraction limited size on a predetermined
recording layer.
[0010] However, since a plurality of recording layers and guide
layer are stacked each other and spaced a predetermined thickness
apart to make up a single grooveless multilayer disc, the distance
between the recording layer and guide layer varies according to
which recording layer is targeted.
[0011] For example, an optical pickup as disclosed in PTL 1 is
configured to converge light with a single objective lens to form
both the light converging spots G and light converging spot R at a
fixed interval therebetween along the optical axis. If the fixed
distance between the light converging spots formed by the pickup is
different from the distance between a target recording layer and
the guide layer in an optical disc, the light converging spot R of
diffraction limited size may be formed on the target recording
layer, but the light converging spots G may not be focused to their
diffraction limit on the guide layer and defocused, resulting in
detection failure of tracking error signals.
[0012] In view of the problem, the present invention has an object
to provide an optical information processing device including a
simply configured optical system and being capable of always stably
detecting tracking error signals for a grooveless multilayer
optical disc in which the distance between the guide layer and
recording layers varies according to which recording layer is
targeted, a tracking control method used in the optical information
processing device, and an optical pickup using the detection method
to deal with the grooveless multilayer optical disc.
[0013] This object can be achieved by the present invention recited
in the scope of the appended claims.
[0014] The present invention can provide an optical information
processing device capable of always stably detecting tracking error
signals for a grooveless multilayer optical disc in which the
distance between the guide layer and recording layers varies
according to the targeted recording layer, a tracking control
method used in the optical information processing device, and an
optical pickup compatible with the grooveless multilayer optical
disc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] These and other features, objects and advantages of the
present invention will become more apparent from the following
description when taken in conjunction with the accompanying
drawings wherein:
[0016] FIG. 1 is a schematic diagram and block diagram showing an
embodiment of the optical information device according to the
present invention;
[0017] FIG. 2 is a schematic perspective view showing a simply
depicted configuration of a grooveless multilayer optical disc and
an example of light converging spots applied on the optical disc
according to the present invention;
[0018] FIG. 3A is the first schematic cross-sectional view of a
relevant disc part to show the state of light spots formed by
converging light beams on respective layers of the grooveless
multilayer optical disc, as an example;
[0019] FIG. 3B is the second schematic cross-sectional view of a
relevant disc part to show the state of light spots formed by
converging light beams on respective layers of the grooveless
multilayer optical disc, as an example;
[0020] FIG. 3C is the third schematic cross-sectional view of a
relevant disc part to show the state of light spots formed by
converging light beams on respective layers of the grooveless
multilayer optical disc, as an example;
[0021] FIG. 4 includes a schematic plan view and block diagram
showing an exemplary configuration of a photodetector and various
kinds of control signal detection circuits according to the present
invention and exemplary states of detected light spots formed on
detection faces and the detection signals;
[0022] FIG. 5 includes a schematic plan view and block diagram
showing an exemplary configuration of the photodetector and various
kinds of control signal detection circuits according to the present
invention, and other exemplary states of detected light spots
formed on detection faces and the detection signals; and
[0023] FIG. 6 includes a schematic plan view and block diagram
showing an exemplary configuration of the photodetector and various
kinds of control signal detection circuits according to the present
invention, and yet other exemplary states of detected light spots
formed on detection faces and the detection signals.
DETAILED DESCRIPTION OF THE EMBODIMENT
[0024] With reference to the drawings, an embodiment of the present
invention will be described.
[0025] FIG. 1 includes a schematic diagram and block diagram
showing an embodiment of the optical information device of the
present invention.
[0026] An optical pickup unit 30, which is one of the components
making up the optical information device, includes a first
semiconductor laser light source 1 for emitting a first light beam
to be converged on a predetermined recording layer in a grooveless
multilayer disc in order to write information signals or read out
the written information signals and a second semiconductor laser
light source 21 for emitting a second light beam to be converged
and applied on a guide layer in the disc in order to detect a
specific tracking error signal. The first light beam 100 (indicated
by dashed lines in FIG. 1) emitted from the first semiconductor
laser light source 1 successively passes through a
wavelength-selective prism 2, a polarization beam splitter (PBS) 3,
a coupling lens 4, a turning mirror (not shown), a quarter-wave
plate 5 and some other optical elements and reaches an objective
lens 6 that in turn converges the first light beam 100 on a
predetermined recording layer in a recording layer group 301
provided in a grooveless multilayer optical disc 300.
[0027] This grooveless multilayer optical disc 300 is composed of
the recording layer group 301 including a plurality of recording
layers stacked each other with a predetermined layer spacing
therebetween and a guide layer 302 that is positioned behind (upper
side in FIG. 1) the recording layer group 301 and provided with a
predetermined guide groove. The light beam 100 is converged on a
predetermined recording layer in the recording layer group 301 by
the objective lens 6 to form a light converging spot R.
[0028] On the other hand, the second light beam 200 (indicated by
solid lines in FIG. 1) emitted from the second semiconductor laser
light source 21 is dispersed by a given amount by an auxiliary lens
22 and then enters a holographic grating 23 that in turn splits the
second light beam 200 into three light beams in total, namely a 0
order light beam passing through the grating 23 without suffering
any diffraction and diffracted +1 order and -1 order light beams,
and the diffracted light beams travel in predetermined directions,
respectively.
[0029] During diffraction, the holographic grating 23 having a
specific grating pattern of unequally-spaced curved grooves imparts
positive and negative lens power conjugated to each other with
respect to the 0 order light beam to the .+-.1 order diffracted
light beams, respectively. One of the +1 and -1 order light beams
is emitted in a relaxed dispersion state with respect to the 0
order light beam, while the other in an enhanced dispersion
state.
[0030] After being diffracted and split by the holographic grating
23, the three light beams 200 are reflected off by the
wavelength-selective prism 2 and are routed through almost the same
optical path as the light beam 100 passing through, in other words,
the light beams 200 pass through the PBS 3, coupling lens 4,
turning mirror (not shown), and quarter-wave plate 5 in succession
to reach the objective lens 6 that in turn converges the light
beams 200 onto the grooveless multilayer optical disc 300 as does
the light beam 100. The three light beams 200 respectively form
three individual light converging spots G on the guide layer 302 in
the disc 300.
[0031] Detailed descriptions about the light converging spots R and
G will be given later.
[0032] After forming the light converging spots R and G on the
predetermined recording layer and guide layer in the recording
layer group 301 of the optical disc 300, the light beams 100, 200
are reflected off by the respective disc layers to become returning
light beams that then enter the objective lens 6 again.
Subsequently, the returning beams are routed through almost the
same optical path as the advancing light beams having passed
through, and then enter the PBS 3 that in turn reflects the
returning light beams to direct them to a compound prism 7. The
compound prism 7 including a wavelength-selective mirror face that
splits the returning light beams into the returning light beam from
the light converging spot R and the returning light beams from
light converging spots G and directs them to different optical
paths.
[0033] Among the light beams, the returning light beam from the
light converging spot R passes through the wavelength-selective
mirror face and enters a detection face 91 in the photodetector 9
via a cylindrical lens 8 that introduces a predetermined amount of
astigmatism to the light beam.
[0034] Note that the astigmatism introduced by the cylindrical lens
8 will be used to generate a focus control signal from a detection
signal obtained by the detection face 91 based on an astigmatic
method, which will be described later.
[0035] Among the light beams, the returning light beams from the
light converging spots G are reflected off by the
wavelength-selective mirror face and further reflected off by a
reflecting face for changing the optical path. Subsequently, the
returning light beams enter a detection face 92 separately disposed
from the detection face 91 in the photodetector 9.
[0036] The configuration of the detection faces 91 and 92 in the
photodetector 9 and the specifics of the signal detection method
will be described later.
[0037] A signal detected by the detection face 91 is fed to a focus
control signal generating circuit 501, a recording-layer tracking
error signal generating circuit 502 and a signal reproduction
circuit 504.
[0038] A signal detected by the detection face 92 is fed to a
guide-layer tracking error signal generating circuit 503.
[0039] The focus control signal generating circuit 501 generates a
focus control signal that is then converted into a focus actuator
driving signal by an actuator driving circuit 506. The focus
actuator driving signal is fed to a lens actuator 10 provided to
drive the objective lens 6 two-dimensionally, thereby controlling
the focus of the objective lens 6.
[0040] The recording-layer tracking error signal generating circuit
502 generates a first tracking error signal that is used to read
out an information signal that has been already written in the
recording layer. As with the case of the focus control signal, the
first tracking error signal is converted into a tracking actuator
driving signal in the actuator driving circuit 506 and then fed to
the lens actuator 10 to perform tracking drive control of the
objective lens 6. This tracking control allows the light converging
spot R to properly trace recording tracks composed of information
signal arrays on the recording layer. As a result, the recorded
information signals are correctly detected and output as reproduced
signals by the signal reproduction circuit 504.
[0041] On the other hand, the guide-layer tracking error signal
generating circuit 503 generates a second tracking error signal
that is used to write a new information signal on an empty
recording layer.
[0042] As described above, the light converging spots G and light
converging spot R are formed by converging light beams by a single
objective lens 6 onto the guide layer 302 and the predetermined
recording layer of the recording layer group 301 in the optical
disc 300.
[0043] In short, the second tracking error signal generated by the
guide-layer tracking error signal generating circuit 503 is fed to
the actuator driving circuit 506 to convert it into a tracking
actuator driving signal and is then fed to the lens actuator 10 to
perform tracking drive control of the objective lens 6, thereby
performing tracking control of the light converging spots G formed
on the guide layer in the optical disc. Following the light
converging spots G enables simultaneous tracking control of the
light converging spot R converged on the recording layer.
[0044] The tracking error signal to be fed to the actuator driving
circuit 506 is selectively switched between the first and second
tracking error signals by a switch circuit 505.
[0045] The output power of the laser light source 1 for recording
layers and the laser light source 21 for the guide layer is
controlled by a laser driving circuit 507 based on laser output
monitoring signals obtained from a laser output monitor (not
shown).
[0046] The operations of the focus control signal generating
circuit 501, recording-layer tracking error signal generating
circuit 502, guide-layer tracking error signal generating circuit
503, signal reproduction circuit 504, switch circuit 505, laser
driving circuit 507 and some other components are always controlled
by a given control circuit 500.
[0047] Referring to FIG. 2, the states of the light converging
spots R and G formed on layers in the multilayer optical disc 300
will be described.
[0048] FIG. 2 is a schematic perspective view showing an example of
a specific structure of the grooveless multilayer disc and an
example of states of the light converging spots R and G formed on
the disc.
[0049] In FIG. 2, like components are denoted by like numerals as
of FIG. 1. Note that the disc is flipped vertically with respect to
that in FIG. 1 for clarity.
[0050] Actually, the grooveless multilayer disc 300 includes a
recording layer group 301 composed of a plurality of recording
layers stacked each other with a predetermined layer spacing
therebetween and a guide layer 302 with a predetermined guide
groove or pit arrays arranged at predetermined intervals in a
radial direction (X-axis direction) of the disc and extending in a
tangential direction (Y-axis direction); however, for clarity, FIG.
2 shows only one recording layer extracted from the middle of the
plurality of recording layers making up the recording layer group
301 and represents the recording layer as a recording layer
301.
[0051] A first laser light beam 100 emitted from the semiconductor
laser light source 1 travels through the predetermined forward
optical path and then is converged by the objective lens 6 on the
recording layer 301 in the multilayer optical disc 300 to form a
light converging spot 101. This light converging spot 101
corresponds to the light converging spot R.
[0052] On the other hand, a second laser light beam 200 emitted
from the semiconductor laser light source 21 is split by the
holographic grating 23 into three light beams, a 0 order light beam
and .+-.1 order diffracted light beams as described above. The
split light beams travel through the predetermined forward optical
path and then enter the objective lens 6 that in turn converges the
beams on the guide layer 302 in the multilayer optical disc 300 to
form three light converging spots 201a, 201b, 201c. These three
light converging spots 201a, 201b, 201c correspond to the light
converging spots G.
[0053] The three light converging spots 201a, 201b, 201c are light
converging spots derived from the light beams diffracted and split
by the aforementioned holographic grating 23. Among the three, the
middle light converging spot 201a is a light converging spot
derived from the 0 order light beam having passed through the
holographic grating 23 without being diffracted. The light
converging spots 201b, 201c, which are located so as to sandwich
the light converging spot 201a from the front and back along the
guide groove on the guide layer 20, are light converging spots
derived from the .+-.1 order diffracted light beams, respectively,
diffracted and split by the holographic grating 23. The light
converging spots 201b, 201c are defocused by a predetermined amount
in an opposite direction to each other with respect to the middle
light converging spot 201a along the optical axis direction (Z-axis
direction in FIG. 2).
[0054] In other words, the light converging spots 201b, 201c have
their diffraction limit within a plane (Y-Z plane) formed in the
optical axis direction (Z-axis direction in FIG. 2) and the
tangential direction (Y-axis direction in FIG. 2) of the disc and
at positions at almost the same distance apart in an opposite
direction from the diffraction limited position (position where the
smallest light converging spot is formed) of the light converging
spot 201a as a center.
[0055] As shown in FIG. 2, the relative distance between the light
converging spot 101 and light converging spot 201a is adjusted so
that, when the diffraction limit of the light converging spot 101
is positioned just on the recording layer 301 (hereinafter, this
state is referred to as "just focus"), the diffraction limit of the
light converging spot 201a is concurrently positioned on the guide
layer 302. This adjustment automatically permits the light
converging spots 201b and 201c to be applied on the guide layer 302
with a predetermined amount of defocus in the opposite direction to
each other.
[0056] The example in FIG. 2 shows the light converging spots 201a,
201b, 201c equidistantly formed along the direction of the guide
groove of the guide layer 302, or in the tangential direction
(Y-axis direction in FIG. 2) of the disc; however, the present
invention is not limited thereto.
[0057] As long as the light converging spots 201a, 201b, 201c are
not formed on top of each other, the spots can be formed anywhere
on the guide layer 302.
[0058] In addition, the number of the light converging spots G
formed on the guide layer according to the present invention is not
limited to three as shown in the embodiment of FIGS. 1 and 2. For
example, with a holographic grating 23 having a predetermined
diffraction efficiency obtained by modifying the cross-sectional
shape of the grating grooves of the grating 23, not only the .+-.1
order diffracted light beams, but also higher order diffracted
light beams can be obtained from the second light beam 200. There
is no problem to increase the number of the light converging spots
G formed on the guide layer 302, to five, seven, nine or more.
[0059] In addition, the optical element to form the plurality of
light converging spots G is not limited to the holographic grating
as shown in FIG. 1. Any optical elements can be used as long as the
optical elements have the ability to form the light converging
spots on the guide layer that are defocused with respect to each
other by a predetermined amount in the optical axis direction.
[0060] Furthermore, FIG. 2 shows the example in which the guide
layer 302 includes a continuous guide groove with a given space
between adjacent groove parts along the radial direction (X-axis
direction) of the disc; however, the present invention is not
limited thereto. There is no problem to use a guide layer 302
including, for example, predetermined pit arrays rather than the
aforementioned continuous guide groove.
[0061] FIGS. 3A to 3C are schematic cross-sectional views of
relevant disc parts to show the state of light spots converged on
the respective layers, more particularly the state of a light
converging spot 101 for recording layers formed on a predetermined
recording layer in the recording layer group 301 of the multilayer
optical disc and three light converging spots for the guide layer,
namely the light converging spots 201a, 201b, 201c formed on the
guide layer 302, as the example shown in FIG. 2.
[0062] For the sake of clarity, FIGS. 3A to 3C show a recording
layer group 301 with three recording layers L0, L1, L2 located in
this order from the furthest with respect to the objective lens 6;
however, the present invention does not of course limit the number
of the stacked recording layers. There is no problem to apply the
invention to optical discs including four or more recording
layers.
[0063] As with the case of FIG. 2, FIGS. 3A to 3C also show discs
flipped vertically with respect to that in FIG. 1.
[0064] FIG. 3A replicates the state of the light converging spots
described with FIG. 2. Specifically, the recording-layer light
converging spot 101 is just focused on a recording layer L1 in the
middle of the recording layer group 301, while a light converging
spot 201a in the middle of the three guide-layer light converging
spots 201a, 201b, 201c is mostly just focused on the guide layer
302. The other guide-layer light converging spots 201b and 201c
strike the guide layer 302, but are defocused by a predetermined
amount. More specifically, the diffraction limit of the converged
light 201b is positioned further (lower side in FIG. 3A) than the
guide layer 302 from the objective lens 6, and the diffraction
limit of the converged light 201c is positioned closer (upper side
in FIG. 3A) than the guide layer 302 to the objective lens 6.
[0065] FIG. 3B shows an example in which the recording-layer light
converging spot 101 is just focused on the recording layer L2 which
is the closest (top recording layer in FIG. 3B) to the objective
lens 6 in the recording layer group 301. In this case, among the
three guide-layer light converging spots 201a, 201b, 201c, the
right converging spot 201b is mostly just focused on the guide
layer 302. The other guide-layer light converging spots 201a and
201c strike the guide layer 302, but are defocused by a
predetermined amount so that the diffraction limits of both the
light converging spots 201a and 201c are positioned closer (upper
side in FIG. 3B) than the guide layer 302 to the objective lens
6.
[0066] FIG. 3C shows an example in which the recording-layer light
converging spot 101 is just focused on the recording layer L0 which
is the furthest (lowest recording layer in FIG. 3C) from the
objective lens 6 in the recording layer group 301. In this case,
among the three light converging spots 201a, 201b, 201c in FIG. 3C,
the left light converging spot 201c is mostly just focused on the
guide layer 302. The other guide-layer light converging spots 201a
and 201b strike the guide layer 302, but are defocused by a
predetermined amount so that the diffraction limits of both the
light converging spots 201a and 201b are positioned further (lower
side in FIG. 3C) than the guide layer 302 from the objective lens
6.
[0067] FIGS. 4, 5 and 6 are schematic plan views showing the
configuration of the photodetector 9 disposed in the optical pickup
30. FIGS. 4, 5 and 6 also show a relevant optical pickup part to
explain how the detection faces in the photodetector 9 are
irradiated with light beams and what kind of signals are detected
by the photodetector 9 according to the states where the light
converging spots are formed on the multilayer optical disc 300 as
shown in FIGS. 3A to 3C.
[0068] Through FIGS. 4 to 6, like components are denoted by like
numerals as of FIG. 1.
[0069] The photodetector 9 includes a photodetection face 91 on
which a light beam reflected from the light converging spot 101
formed on the recording layer of the disc is converged as a
returning light beam 102 and a photodetection face 92 composed of
three independent photodetection faces 92a, 92b, 92c on which light
beams reflected from the light converging spots 201a, 201b, 201c
formed on the guide layer are converged as returning light beams
202a, 202b, 202c, respectively.
[0070] The photodetection face 91 is, for example, divided into
four quadrants by crossed parting lines as shown in the FIGS. 4 to
6. Detection signals respectively obtained from the divided
detection faces are supplied to a focus control signal generating
circuit 501, recording-layer tracking error signal generating
circuit 502 and signal reproduction circuit 504.
[0071] The detection signal supplied to the focus control signal
generating circuit 501 is converted into a focus control signal by
an astigmatic method. This control signal is output to adjust the
focus of the objective lens 6.
[0072] The tracking error signal generating circuit 502 outputs a
tracking error signal (DPD signal) converted by a differential
phase detection (DPD) method. This DPD signal is used to perform
tracking control of the objective lens 6 to reproduce the recording
layer.
[0073] The signal reproduction circuit 504 outputs a reproduced
signal from the recorded recording layer.
[0074] The method of detecting the focusing and tracking error
signals, the method of reproducing the recorded information signals
and detection principle thereof are well known in the art, and
therefore detailed descriptions thereof are not reiterated.
[0075] The present invention does not limit the method of detecting
the focus control signals and tracking error signals to the
aforementioned astigmatic method and DPD method.
[0076] When a new information signal is written in an empty
recording layer, signals detected on the photodetection face 92 are
fed to the guide-layer tracking error signal generating circuit 503
that in turn generates a tracking signal of the light converging
spots for the guide layer. The tracking signal is used to perform
tracking control of the objective lens 6.
[0077] Each of the three independent photodetection faces 92a, 92b,
92c, which make up the photodetection face 92, is divided into two,
an upper segment and a lower segment as in FIGS. 4 to 6, by a
straight parting line extending roughly along a direction (Y-axis
direction in FIGS. 4 to 6) corresponding to the tangential
direction of the optical disc. Signals from the respective two
segments undergo a subtraction operation by subtractors 503a, 503b,
503c in the guide-layer tracking error signal generating circuit
503. Thus, independent tracking error signals (push-pull signals)
are obtained by a push-pull method from returning light beams 202a,
202b, 202c respectively corresponding to the guide-layer light
converging spots 201a, 201b, 201c. The method of detecting the
tracking error signals with the push-pull method and principle
thereof are well known in the art and will not be described in
detail.
[0078] As described above, in FIG. 3A, only the middle light
converging spot 201a is mostly just focused on the guide layer 302,
while the other light converging spots are defocused by a
predetermined amount. In FIG. 4, the returning light beam 202a
corresponding to the light converging spot 201a enters the
photodetection face 92a and results in a push-pull signal of the
largest amplitude and high quality.
[0079] On the other hand, push-pull signals obtained by the other
photodetection faces 92b, 92c have extremely small amplitude in
comparison with the push-pull signal obtained by the photodetection
face 92a.
[0080] When these push-pull signals undergo an addition operation
in an adder 503d, the resultant signal becomes a tracking error
signal of as good a quality as the push-pull signal obtained by the
photodetection face 92a. Using the signal subjected to the addition
operation enables proper tracking control of the objective lens
6.
[0081] FIG. 5 shows the state of the photodetector 9 when only the
right light converging spot 201b is mostly just focused on the
guide layer 302 as shown in FIG. 3B. In this case, the returning
light beam 202b corresponding to the light converging spot 201b
enters the photodetection face 92c and results in a push-pull
signal of the largest amplitude and high quality. As with the case
shown in FIG. 4, push-pull signals obtained by the other
photodetection faces 92a, 92b have extremely small amplitude in
comparison with the push-pull signal obtained by the photodetection
face 92c.
[0082] When these push-pull signals undergo an addition operation
in the adder 503d, the resultant signal becomes a tracking error
signal of as good a quality as the push-pull signal obtained by the
photodetection face 92c. Thus, as with the case shown in FIG. 4,
using the signal subjected to the addition operation enables proper
tracking control of the objective lens 6.
[0083] The same thing as FIGS. 4 and 5 can be applied to FIG. 6.
Specifically, FIG. 6 shows the state of the photodetector 9 when
only the left light converging spot 201c is mostly just focused on
the guide layer 302 as shown in FIG. 3C. In this case, the
returning light beam 202c corresponding to the light converging
spot 201c enters the photodetection face 92b and results in a high
quality push-pull signal of the largest amplitude. As with the case
shown in FIG. 4, push-pull signals obtained by the other
photodetection faces 92a, 92c have extremely small amplitude in
comparison with the push-pull signal obtained by the photodetection
face 92b.
[0084] When these push-pull signals undergo an addition operation
in the adder 503d, the resultant signal becomes a tracking error
signal of as good a quality as the push-pull signal obtained by the
photodetection face 92b. Thus, as with the case shown in FIG. 4,
using the signal subjected to the addition operation enables proper
tracking control of the objective lens 6.
[0085] As described above, whatever recording layer in the
recording layer group 301 the light converging spot 101 is just
focused on, a good tracking error signal can be constantly detected
from the guide-layer light converging spot formed on the guide
layer 302. Performing the tracking control of the objective lens 6
with the tracking error signal allows proper tracking control of
both the light converging spot 101 for recording layers and light
converging spots for the guide layer.
[0086] The embodiment described with FIGS. 3A to 3C and FIGS. 4 to
6 indicates an example in which at least one of the light
converging spots to be formed on the guide layer 302 is just
focused on the guide layer 302 whatever recording layer in the
recording layer group 301 the recording-layer light converging spot
101 is just focused; however, the present invention is not limited
thereto.
[0087] There is a possible case where the number of the recording
layers is greater than the number of the light spots to be
converged on the guide layer. In this case, when a recording-layer
light converging spot 101 is just focused on a predetermined
recording layer, none of the guide-layer light converging spots may
be just focused on the guide layer. However, as shown in FIGS. 4 to
6, adding up the push-pull signals obtained from the respective
light converging spots for the guide layer brings a signal almost
equal to the high quality push-pull signal having the largest
amplitude among the signals, thereby constantly providing a good
tracking error signal.
[0088] In addition, the present invention is not limited to the
method of adding up the push-pull signals obtained from the
respective guide-layer light converging spots.
[0089] There is no problem of not performing the addition operation
on purpose. One of the possible methods includes individually
monitoring push-pull signals obtained from guide-layer light
converging spots and choosing a high quality push-pull signal of
the largest amplitude when needed to use it as a tracking error
signal.
[0090] The embodiment shown in FIG. 1 and FIGS. 4 to 6 shows an
example in which the tracking error signal is detected from the
guide-layer light converging spots by the push-pull method;
however, the present invention is of course not limited thereto.
There is no problem of adopting other well-known focusing and
tracking error signal detection methods.
[0091] For example, provision of predetermined pit arrays to the
guide layer 302, instead of the continuous guide groove, allows
detection of tracking error signals from the guide-layer light
converging spots 201a, 201b, 201c by the DPD method as with the
case of the recording-layer light converging spot 101. The DPD
method can advantageously avoid tracking error signal offset caused
by objective lens displacement when using the push-pull method.
[0092] Even when using the DPD method, adding up DPD signals
obtained from the guide-layer light converging spots or choosing
one DPD signal as described above enables constant proper tracking
control whatever recording layer the light converging spot is just
focused on among the plurality of recording layers.
[0093] Furthermore, the present invention is not limited to the
optical information device with the configuration shown in FIG. 1.
As long as the optical information device is configured to form
light spots, which are defocused with respect to each other along
the optical axis direction, on a guide layer in a grooveless
multilayer optical disc to detect independent tracking error
signals respectively from the guide-layer light converging spots,
the invention can be applied to any types of optical information
devices.
[0094] While we have shown and described an embodiment in
accordance with our invention, it should be understood that the
disclosed embodiment is susceptible of changes and modifications
without departing from the scope of the invention. Therefore, we do
not intend to be bound by the details shown and described herein
but intend to cover all such changes and modifications that fall
within the ambit of the appended claims.
* * * * *